Inductive artificial line



Dec. 1, 1931. M. l. PURIN INDUCTIVE ARTIFICIAL LINE Filed Feb. 24, 1928 4 Sheets-Sheet 3 MICHAEL I. PUPIN 5mm zb w Dec. 1, 1931.

M. I. PUPlN INDUCTIVE ARTIFICIAL LINE Filed Feb. 24, 1928 4 Sheets-Sheet 4 FIG. 6,

FIG. 5.

' FIG. 7

FIG. 6

FIG. 9

FIG. IO

INVENTOR MICHAEL l. PUPIN ATTORNEY Patented Dec. 1, 1931 ENT: oFFlce MICHAEL invonsxY Perm, 0F NORFOLK, CONNECTICUT INDIITCTIVVE ARTIFIG'IAL LINE Application filed February 24, 1923. Serial 'No. 256,571." r

The object of my invention is to provide an improved method for duplex transmission of'electrical signals over wave conductors, as for instance, over suboceanic cables.

. Theordinarypractice of duplex signalling over long submarine cables employs atthe transmitting station a local sectional wave conductor, the socalled artificialline, whichis placed on one side of a duplex bridge, the other side of the bridge containing the cable. The electrical currents generated by the electrical pulses impressed upon the two sides of the bridge by a'local generator are split into two parts; one part is transmitted over the,

side containing the cable and the other over the side containing the local artificial line. hen the cable and its balancing artificial line have the same terminal resistance and reactance for every frequency which 1s of 1mportance at the speed of signalling then they can be made to balance each other. It isobvious that when the local wave conductor-is an ordinary artificial line consisting of resistances in series and capacities in parallel then at higher speeds of signalling such a balance cannot be obtained, because such an artificial line is non inductive, whereas the cable has inductance the reaction of which becomes a determining factor-flat higher speeds. Inductance introduced into the artificial line Wlll not improve the balance, unless this inductance and its internal resistance vary with the frequency according to the same law as the average inductance and resistance of the cable elements. The process of formulating the rules for designing such an 1nduct1ve artificial line 1s d sclosed in this specification and it consists of two distinct mathematical formulae which guide the' design. 1'

' have discovered that: the average eifecand resistance marine cable balanced "for duplex operation tive resistance and inductance of a submarine cable vary as though each were comprised oftwodistinct parts, one constant, and one varying with the frequency, and that the variable part'of the inductance decreases whereas the variable part of the resistance increases with the frequency of the impressed vary with the frequency of the impressed current in the same or, substantially the same manner as the average effective inductance go and resistance of a submarinecable vary. This improved network furnishes therefore a means for accurately balancing a submarine cable over awide range of frequencies.

My invention comprises an inductive arti- .05 ficial line, preferably in the form of equal sections connected in series, the inductance. and resistance of each section of which vary in accordance with the laws which I have discovered and above described. 1-70 In the drawings accompanying and forming part of this specification and in which like reference numerals designate correspending parts throughoutz- Fig. 1 is a diagrammatic representation by 7 curves A, B and C of the effective terminal reactances and resistances of a submarine cable and of. a balancing artificial line.

Fi g. 2 is a diagrammatic representation by curves A. and B of the effective inductances per nautical mile of the same cable. p 1

Fig. 3 diagrammatically represents the constructional elements of a sectionalartifi- 85 Icial line madein accordance With my invenytion;

Fig. 4: diagrammatically represents a subby means of my invention p 90 Fig. 5 is a plan view of an air-cored coil of toroidal symmetry.

Fig. 6 shows an end view of the same.

Fig. 7 is a sectional view of the same taken on the line 77 of Fig. 6.

Fig. 8 is a sectional view of the coil shown in Figs. 5, G and 7 after an insulating compound has been poured around it.

Fig. 9 is a plan view of a nickel cored coil of toroidal symmetry.

Fig. 10 is a sectional view of the same taken on the line 10-10 of Fig. 9.

Referring to the diagram of Fig. 1, curves A and B represent at various frequencies the effective terminal resistance a and reactance b respectively, of an actual submarine cable which has the following constants:

Length l==974.33 nautical miles Capacity C=0.38l 10* farads Resistance R=2.l5 ohms Estimated inductance L:7.4 10- hvnrys -Where 2 capacity per nautical mile L =effective average inductance per 11. m. at frequency f R =effective average resistance per 11. m. at frequency f From Equation (1) follows:

From curves A and B of Fig. 1, curves A and B of Fig.2 were calculated by Equations The latter curves, derived by calculation from experimental measurements give L and R and they represent graphically the law of variation of the average effective resistance and of the average effective inductance, respectively, per nautical mile ofthe cable. They furnish the data for the design of the inductive artificial line disclosed. by this invention. If, therefore, a sectional artificial line can be constructed in which the effective resistance and the effecsufficiently broad frequency interval the same or very nearly the same as in the cable then the two structures will have the same or very nearly the same terminal impedance for that frequency interval, and can balance each other in the duplex bridge.

The sectional artificial line described here is a structure of this kind as the following 1n athematical analysis will show. It is made up of preferably equal sections, each section having two inductance coils in series, one of which is shunted by a non-inductive resistance. At each point of uncture of two consecutive sections a condenser of suitable capacity is inserted between this point and the ground. Fig. 3 gives a graphical representation of one of these sections. The two inductance coils 3 and 4 are connected in series, coil 4 being shunted by a non-inductive resistance 7. Equal condensers and 6 are connected to points of juncture, 1 and 2, and to the ground through 8. Let each of these sections be an equivalent of nautical miles of the cable; hence condensers 5 and 6 at the two terminals of the section will each have a capacity of 3.84 10" farads. The total capacity of the artificial line should, preferably, be equal to the total capacity of the cable. The length of one will then be an electrical equivalent to the length of the other.

Let L and R be the effective inductance and resistance, respectively, of the shunted coil for frequency f lOS; then it can be shown that lVhere R=internal resistance 1 Il -internal inductance J of C011 R shunt resistance of the same coil.

consideration. Thus:

L 1a L1a L lf+ 403 R0 T: 960. etc

give the effective inductances and resistances, respectively for the frequencies 20, 30, etc.

tive inductance per nautical mile are for a These values L L etc. and R R etc.,

can be made equal to the average effective inductances L L" etc. and average effective res stances R" RQ etc. of a 10 nautical miles length of the cableprovided that another coil of a definiteinductance L and resistance B be connected in series with-the shunted inductance as indicated in Fig." 3. With this addition, which is indispensible, Formulae (3b) and (4b) become now Where 11 0, 11 30, etc. and R,20,4R30, etc. are, the effective average inductances and resistances, respectively,-of the cable, per nautical mile, as calculated from terminalimpedance measurements by Formulae (4), and given in curves A and B of Fig. 2. If Formulae (3c) and (40) represent inductances and resistances varying with the frequency in the same manner as the effective average inductances and resistances'of'a cable length of 10 nautical miles, then the following conditions must be satisfied:

That is, if the curves represented (3d) and (4d) are to coincide over a suitablefrequency interval with curves A and B of Fig 2 then they certainly must coincide at the points corresponding to frequencies 2O'and 30. These frequencies have been selected",

' because they are the characteristicfrequencies in the case of a cable which Works at a speed of 600 letters per minute, as is the'case in the cable which is consideredhere. The.

values calculated from the mathematical Formulae (3d) and (ad) for the effective inductance and resistance,respectively, at frequencies 20 and 30 Will be the same as those derived from experiment for these frequen'-" cies, if Equation (9) "is fulfilled; it willlbe fulfilled if for a suitably selected value, of

, inductance L we select proper values for R 'and R since g is a constant, which has a definite value assigned to itby experimental measurement. The selection, hQWBVGIB lHHSiI besuch that not only is Equation (9) satisfied, but also Equations (5),,(6), 7), and.

(8).: Thiscan bedone-asfollows:- 1

Hence the co'nstant a just like 9, is fixed 'by the experimental measurements of the'impedances at f==20 and f=30. It will. be shownnow that the value offL}, is also fixed by these measurements. From (5) and, (6) follows: i

a 10L' L,, 1+9a.,

which gives L in terms of the experimentally determined constants 10L'20, 10L'. and a That is to say, inductance L is fixed by the experimental measurements selected arbitrarily.

Again from 5 t follows that so that having a coil :of inductance L, suitably selected, fixes the value of the constant a. But no matter how we select the value of L the product L(1-a) 2 must have a definite value. From (4a) follows:

We also have R+R Therefore 1 V- 21r 10L( 1-a) With a given coil of inductance L the value I of the shunt resistance R is therefore fixed.

so that the internal resistance R of the shunted coil is also fixed. The only remaining constant to be determined is R it cannot be emf,

From (7 follows Therefore 1 (a+4a )R a=0.3865 I 1 R0 1 1 L 1 a 56.62 millihenrys R This shows that the two impedance nieasure- 0.63

ments at frequencies and and the selection of the value of L determine in a perfectly definite manner all of the constants in the fundamental Equations (Sci) and (4d) Nothing is arbitrary except the selection of the value of'L. In other Words, if a sectional artificial line with shunted inductance in its sections is to imitate a submarine cable which is to be balanced by it then the theory and the experimental measurements described here iiurnishfl a complete and. unique information for the design of these sections. The following numerical calculations will illustrate this:

= q 1 09 (very nearly) From Equation (11) If we make L=0.15, then (12) and (13),

from Equation ?8 5'". I'X R 1964+ 2) P z 19 84+ (3365+ 9x A From Equation There remains only one constant to deter mine, that 1s R Equation (15) gives (a 4a.?) R (1 411 It Will be shown now how the effective inductances and resistances calculated from For- Hence mulae (3d) and (4d) compare with those obtained from cable measurements and recorded in curves A and B, oi Fig. 2.

L(l-a) =56.62 1O All these inductances are in millihenrys.

=2734 (27.34) by choice The numerals in bracketsare'those from curves A and B, of Flg. 2. The results calculated from theformulae of the mathematical theory disclosed here are, therefore, 1n

splendid agreement with the observed results for a band of frequencies between f 5, and

f 50. This is the band which includes all the-important frequencies in ordinary cable signalling at the speed of 600 letters per min ute; The curves plotted from these calculated data indicated by dots in Fig. 2.

practically coincide with curves A and B of Fig. 2. Their deviation is visible on account of the large scale employed in the 'diagram' of this. figure. V

If,.ho.wever, from these calculated Values of the effective inductances and resistances oi the artificial 7 line per I section its 7 terminal re actances and resistances are calculated and the curves plotted then their 'devlations from curves A and B in Fig. 1, are scarcelyvis ble'.

' The a v p points these curves are given by the dots lll tlll s figure. Differences canbe detected by veryacurate impedance measure finents, and these differences are found to be" extremely small; they a-re not detectableat all in the vicinity offrequeney 20, which is the characteristic frequency at thespeed of 600 letters per minute. Calculation shows that even at frequencies 5 and 50 these differ- I ences are very small. Thus ifwe denote lby ,0 and (L Z2 5. and b the terminal resistances and reactances, respectively, at frequencies 10, 40, and we shall have 11 :2 17 (248) a =137.4 ssya zss (125) 'b sfziz 212 enemas 08.5) taxation The numerals in brackets denote the values obtained by lvheatstone' bridge measurements, and recorded in curves A and B Fig. 1. Even at frequency f==5 the agreement between the terminal rcsist'ance and reactance of the inductive artificialline described here is to within less than 1% withthose obtained on the cable by lVheatstone bridge measurements. i

V The remarkable agreement between the the same curves of variation as the eliective resistance and inductance of a suitably shunted inductance coil, and this is taken care of by the shunted coilin each section. The

curves which exhibit thisphysical fact must be-determined by l/Vheatstone bridge measurements explained, here! They are the physical basis ofthis invention. Previous attempts to construct an inductive artificial line which imitates its corresponding cable did not as far as applicant isaware recognizethe existance of this physical fact and hence they failed. I

" The cable under consideration here, when in commercial operation, has a length of 974.33 11. In. Since its constants are':'

0: 0.384 X 10- farads R=2 45 Ohms 7 porn. m., at zero trcqucncy.

itis obvious that the signal components corresponding to frequencies smaller than 6 p. p. s. and reflectedat the receiving terminal of the cable will be appreciable when they return to I the transmitting end. This will. disturb the balance if suitable provisions are not made. Hence, having constructed the inductive artificial line in the manner described here, and having made it equivalent to 974.33 11. m. of

tional means, in order tomake the low frequency part of the signal reflected atthe receiving end ofthe cable equal to the corresponding reflected part atthefurthest terminal of the inductive artificial line. This is accomplished by the well known method which has been in practice for many years.

The method consists in attaching to the terminal of the artificial line a network of conductors which has the same impedan'ce for I the low frequencies as the network which is employed at the receiving end of the cable.

Thisris indicated in the diagramofFig. 4. Referring to this 'disgram it represents two cable stations employing the same cable for electrical exchange of messages by the duplex method-of operation, and balanced'with my improved artificial line.

The numerals 1, 2, 3', etc. referto the apparatus' at one end of the cable, and 1', 2", 3, i

etc. refer to similar apparatus at the other end. The apparatus atlone end, only, will be described; the same description applies to the other end. I I 1 denotes. the sending generator; 2, 3 de note two condensers in the two sides of the 'Wheatstone bridge, havingin one of its arms the cable 7 and in the other the] inductive artificial line described above and represented here by coils 11, 12, 13, 14, etc., and condensers 23, 24, 25, etc. I In each section of theinductive artificial line there are twoinductance coils, 11,12, etc., one of which is the cable, it is necessaryto provide addi- Inn shunted by a non-inductive resistance, 19,

etc; The artificial line and the" sending generator are connected as usual through socalled sea earths at 8 and 9 to "ground. An impedance 5 is connected" in series with the cable 10 and serves to balance the impedance -which' the sea-earth 9 places inserie's with the inductive artificial line. ,Numeral' tdenotes the signal receiving apparatus; At the end of the inductive artificial line is a variable capacity 29 anda variable resistanceBO by the adjustments of which their impedance is made approximately equal to the impedance of the network which connects the cable to ground at the receiving station. The receiving apparatus in cross-arm 4 maybe any suitable curbing and amplifying system of conductoi's with local sources of energy supply.

The very object of this invention is, to innalling system described here.

The inductive artificial line described here has been. assumed to consist of equal sections, each section being equivalent to a cable length of 10 n. m. This assumption simplifies the explanation of the structure; it also simplifies its manufacture. But the wellknowii mathematical theory of sectional wave conductors makes it obvious that the sections may be much longer at signalling speeds of 600 letters per minute It is also obvious that the artificial line may be divided into groups of sections 1111Vl1l0l1 each section of one group represents a cable length of say 10 n. 111.; each section of the next group represents a cable length of 15 ii. 111.; and each section of the next group represents a cable length of 20 n. in. or even n. m. It is obvious, however, that each group must have its inductance coils adjusted in accordance with the mathematical theory given above. Mathematical theory demands also that the capacity at the transmitting end of the artificial line must be half as large as the other capacities; that is, it must be 1.92 X 10* farads in the case described above.

It is essential that the inductance coils used in the balancing network have a negligible external field. Otherwise they will have mutual inductance with external circuits and will. therefore be sensitive to external disturbances. This is specified in claim; 7 by the words closed magnetic circuit; the word closed meaning that the lines of force of the magnetic circuit have no inter-linkage with external electrical circuits. A toroidal coil is the simplest illustration of aclosed magnetic field of this kind, but there are other forms of coils with paramagnetic cores which have a negligibly small external field and therefore no mutual inductance with external circuits. Another essential requirement is that the effective inductance and its accompanying effective resistance must not vary appreciably with the intensity of the signaling current. An inductance varying with the strength of the signaling current makes the artifical line unfit for balancing a submarine cable, because the effective average inductance and resistance of ordinary cables do not vary with the strength of the signaling current. Such inductance coils will i be described now.

Fig. 5 represents ten coils 31, 31, etc. wound upon ten wooden blocks 32, 32, etc. Figs. 6 and 7 show these blocks with holes indicated by numerals 3. These holes permit the blocks to be slipped over a metal rod and clamped so as to form a rigid wooden rectangular pattern. The ten blocks forming this pattern are permanently coupled by a flexible coupling consisting of a strip of linen indicated by theblack line 34 in Fig. 7 and by the dotted lines 34 in Fig. 5. Coils 31, 31, etc, are then wound. by a machine and thus it is possible to make the individual coils equal to each other in every respect. \Vhen all the ten coils have been wound the clamping rod is removed from the rectangular pattern and the ten coils, connected inseries and coupled to each other by the linen ribbon, are then distributed symmetrically around a circular wooden cylinder forming a single symmetrical coil consisting of a plurality of individual coils as represented in Figs. 5, 6 and 7. Such a coil may be made to retain its form permanently by pouring around it an insulating compound made fluid by heat which hardens upon cooling. Fig. 8 shows a cross-sectioinil view of the coil after'the insulating compound has been applied. An air-cored coil of this con- L struction has no appreciable external magnetic field except at points in the immediate vicinity of the surface of the winding. The insertion into this part of the magnetic field of a thin sheet of magnetic material like. silicon steel offers a means of a small adjustment of the inductance of the air-cored coil. The air-cored coil acts like an ideal toroidal coil. Tts inductance and resistance do not vary with the strength of the signal- 1 ing current. The inductance coil shown in detail in Figs. 5, (3. 7 and 8 is the inductance coil 3, Fig. 3: and 11. 13. 15, 17, etc., Fig. 4. It is an unshunted inductance coil, and h magnetically neutral core material as iiidicated above.

A similar coil but wound upon a paramagnetic core is shown in Figs. 9 and 10. Fig. 9 is a top view and Fig. 10 represents the cross-secti on of the coil. The core con- 1 sists of circular rings of cold rolled sheet nickel having a permeability of about 11. The plates are about 14 mils thick. The external field of such a coil is vanishingly small. Neither this coil nor the air-cored coil have any mutual inductance with external circuits and, hence, no external electrical disturbances can afiect the circuit of such coils. The nickel cored inductance coil is employed in every section of the inductive artificial line as a shunted inductance, the air-cored coil. being coils, the inductance of which shall not vary, V with the intensity offthe magnetizing force,

the unshunted inductance. The nickel cored inductance coil, shown in detail in Figs. *9 and 10 has a core of paramagnetic material whose permeability varies less with changing magnetizing forces than does that of iron,

and'is the shunted inductance coil 4, Fig. 3

and 12, 14, 16, 18, etc, Fig. 4. i

he nickel cored coil being shunted by re sistance 7 (Fig. 3) receives a part, only, of the signaling current which passes through the air-cored coil, whereby the magnetizing force of this current upon the nickel core is diminished. It can be shown that if A be the amplitude of the signaling current passing through the winding 01 the air-cored coil and B the amplitude of the current which passes through the shunted winding of the nickel cored coil then to one section of the inductive artificial line. It. '?11l(lll0't21116 of the air-cored coil. 7

' L=inductance ot'the nickel cored coil before being shunted.

The intensit of the magnetizin force of B can be easily calculated by the mathematical theory disclosed here, when the amplitude A of the signaling current is given. Ordinarily A=10- in absolute units.

Let the dimensions and the of the nickel core be as follows Outside diameter 9 X 2.54 cm. Inside=6 X 2.54 cm. Height 4 X 2.54 cm.

- Permeability a=11 lVhen A=1O- then a simple calculation by 'varies very little in the interval. between L" 10s==etlective inductancefor frequency 10s, of the cable length which 18 equivalentpermeability 2. In a submarine cable duplex tedegraph system a balancing artificial line comprising a plurality of sections, each section containinga plurality of simple independent inductance coils connected in series, one at least of which is shunted by a non-inductive resistance elements I 3. In a submarine cable duplex telegraph system a-balancing artificial line comprising a plurality of sections, each section containing a plurality of simple independent inductance coils connected in series, one at least of which is shunted by a non-inductive re sistance, and shunt capacity elements.

l. In a submarine cable duplex telegraph system a balancing artificial line comprising a plurality of sections, each section comprising a series arm and a shunt arm and containing in its series arm'two simple and independent inductance coils connected in series, one of which is shunted by a non-inductive resistance element, and containing initsshunt arm a capacity element.

5. Means as set forth in claim 2 further characterized in this, that the unshunted inductance coils have magnetically neutral core material, and theshunted coils have cores of a paramagnetic material the variation in permeability with varying magnetizing forces of which is less than that of iron.

charactmized in this, that the unshunted inductance coils have cores of neutral magnetic material and the shunted inductance coils have nickel cores.

7. Means as set forth in claim 2 further characterized in this, that each inductance coil has a closed magnetic circuit whereby its external magnetic field is minimized.

MICHAEL I. PUPIN.

H=0 and Hi -=1, very much less than that of cold rolled sheet steel or even that of ordi- "nary iron dust cores; hence the desirability of employing nicl-iel cores for the inductance employed in cable telegraphy.

Having thus described the physical and the mathematical.foundation of the invention.

ing a: plurality. of independent series ele-' ments possessmg a magnetic characteristics which in some of these elements are indepenednt of frequnecy and in the remaining elements vary with frequency. 

